It's all just data, isn't it?

Route it all through VoIP and record it there?

Maybe theres some bank-compliance phone vendor that operates its own MVNO and does the recording for you?

here's something like that - https://www.teleware.com/products/mobile-voice-recording/

No it’s not possible in the way or manner you described unless it’s an office phone.

Banks aren’t creating a super secret MVNO to comply with regulations and the telcos aren’t cutting breaks for a bank or a 3rd party spy network. I’d imagine saving all employees cellphone calls for 7 years is a very expensive project.

You can encode voice with quite acceptable quality at 8kbps. (With recent advancements even 3kpbs is reasonably good.[1]) That’s 3.6MB per hour. Assuming an average of one hour per workday on the phone, a single employee generates less than 1GB a year. That’s peanuts. Even eight times that is lower than what I personally generate every day.

[1] https://ai.googleblog.com/2021/02/lyra-new-very-low-bitrate-...